Investigation found two causes
- The foam "seal" where the heater pipes go through into the engine bay is perished and allowing a trickle of water to get in. This may or may not be too difficult to sort out but I have nightmares about removing TR7 heaters so I don't want to think about that too much.
- There's a flipping big hole under the battery tray where the bulkead meets the inner wing - chewing gum won't fix it! That's going to need some proper work with cutting out the battery tray, fabricating new metal patches and then welding them in followed by seam sealer and painting. Flipping 'eck !!!